1. Field of the Invention
The present invention relates to a stretched molding comprising a propylene-ethylene block copolymer as the main component, and a process for producing the same. More particularly, the invention relates to a surface-roughened stretched molding of the kind as described above to which a dull tone has been imparted, and a process for producing the same which comprises stretching a flat molding formed from a composition comprising a propylene-ethylene block copolymer and a specific petroleum hydrocarbon resin added thereto to form an plurality of microvoids on the inside and surface of the flat molding.
The present invention further relates to a heat-shrinkable foamed molding and a process for producing the same. More particularly, the invention relates to a heat-shrinkable foamed molding formed from a specific propylene-.alpha.-olefin copolymer and a specific hydrogenated cyclopentadiene resin having a high softening point, and a process for producing the same.
2. Description of the Prior Art
In general, surface-roughened flat plastic moldings, for example, in the form of a monoaxially or biaxially stretched film, are widely used as polychromatic paper, tracing paper, release paper, general-purpose packaging paper etc.
Conventional processes for roughening the surface of a molding include a filler addition method wherein a large amount of an inorganic filler such as calcium carbonate, silica or sodium silicate is added to a molding material; a post-treatment method wherein a molding is treated with a solvent or a chemical to be dulled; and a coating method wherein a molding is coated with a matte coating film. The filler addition method is defective in that addition of a large amount of an inorganic filler to a molding material is liable to cause sharp reduction in the flowability, during extrusion, of the material. This method results in clogging of a screen bag, failure in obtaining products having a roughened surface with a high density of unevenness because of its liability to cause moisture absorption and foaming of the material attributable to the inorganic filler as well as poor dispersion of the inorganic filler, etc. The post-treatment method is economically disadvantageous because of the necessity of a step of removing a solvent or a chemical from a molding after the treatment thereof with the solvent or the chemical, while the coating method is also economically disadvantageous because of the necessity of investment in coating equipment and facilities related thereto. Furthermore, the latter two methods are defective in that products are very liable to undergo damage due to wear because the improvement attained by these methods is directed only to the surfaces of moldings.
Other known processes for modifying the surface of a film include a lamination method wherein a film of a blend composition comprising polypropylene and high-density or low-density polyethylene is laminated on a polypropylene film, and a lamination method wherein a film of a propylene-ethylene block copolymer alone is laminated on a polypropylene film (see Japanese Patent Publication No. 32,954/1982). However, lamination of either of the above-mentioned two types of films is defective, for example, in that the resulting laminated film has too rough a surface without a high density of unevenness.
Heat-shrinkable foamed moldings in the form of a foamed sheet or a foamed film are used as packing materials, tying materials, materials of labels for containers such as glass bottles,, plastic bottles and metallic cans and materials for protective covers attached to the peripheries or bottoms of bottles to keep the bottles from breaking. The use of such heat-shrinkable foamed moldings allows not only thin-walled bottles to be produced, but also the display effect of beautiful printing to be utilized.
Such foamed moldings have been predominantly formed from polystyrene as the starting material thereof. Since polystyrene is weak and brittle against impact due to the characteristics thereof as a starting material. However, stretching of polystyrene in the course of production of a heat-shrinkable foamed molding is liable to entail fracture thereof at around the grips of a tenter and the like because of the brittleness inherent in polystyrene. Furthermore, it is widely known that foamed moldings of polystyrene obtained as products involve disadvantages including breakage thereof during the course of a bottle-washing step, a filling step, a conveying step, etc. when they are used in the form of a label attached to a bottle or the like, and failure in preventing dangerous scattering of shards of a bottle broken upon dropping because of the brittleness of polystyrene when they are used in the form of a protective cover for the bottle.
Foamed moldings proposed with a view to obviating the disadvantages of such foamed polystyrene moldings include laterally-stretched foamed moldings, heat-shrinkable in the lateral direction, which is formed from a blend composition comprising an ethylene-propylene copolymer containing propylene monomer units as the main component, a specific elasticity-modifying polymer and a decomposition type foaming agent (see Japanese Patent Publications Nos. 22,787/1987 and 33,251/1987).
There has further been proposed an heat-shrinkable foamed film formed from a blended composition comprising a propylene type polymer, high-density polyethylene and a decomposition type foaming agent (see Japanese Patent Laid-Open No. 13,440/1987).
These known foamed moldings all went through melt extrusion at a high temperature of at least the melting or softening point of each corresponding composition to make the decomposition type foaming agent form cells of foam, however, they are defective, for example, in that the cells of foam therein are not uniform, and in that there is difficulty in controlling the conditions of production thereof as can be seen from the tendency of a raw film to be broken or too roughened on the surfaces thereof by stretching thereof to permit beautiful printing on the resulting product.
Furthermore, these moldings incur problems including a lower heat shrinkability at the time of forming than foamed polyestyrene moldings. These defects and problems have been serious obstacles to the expansion of use of propylene copolymers.